Question

I’m writing an Application where a user can upload .rdf-Files and then execute SPARQL-Queries on it. Right now I am stuck on how to format the query’s result. For example: an ASK-Query outputs a boolean and a SELECT-Query returns a ResultSet. CONSTRUCT and DESCRIBE return a new RDF-Graph.
How do I get the result of the given query in a form to pass it to my view? In there I hope to print out a HTML-Table.

For a better understanding of my Problem I created this minimalistic code example:

import java.io.InputStream;

import com.hp.hpl.jena.*;

public class playground {

    public static void main(String[] args) {

      InputStream in = FileManager.get().open("vc-db-1.rdf");
      Model model = ModelFactory.createDefaultModel();
      model.read(in, null);

      String queryStringSelect = "SELECT ?subject ?predicate ?object WHERE { ?subject ?predicate ?object }";
      String queryStringAsk = "ASK WHERE { ?subject ?predicate ?object }";
      String queryStringDescribe = "DESCRIBE * WHERE { ?subject ?predicate ?object }";
      String queryStringConstruct = "CONSTRUCT { ?subject ?predicate ?object } WHERE { ?subject ?predicate ?object }";

      QueryExecution qe = QueryExecutionFactory.create(queryStringDescribe,
        model);
      Query q = QueryFactory.create(queryStringDescribe);

      int queryType = q.getQueryType();
      switch (queryType) {
      case Query.QueryTypeAsk:
          boolean b = qe.execAsk(); // Result that has to be formatted
          ResultSetFormatter.outputAsTSV(System.out, b);
          break;
      case Query.QueryTypeConstruct:
          model = qe.execConstruct(); // Result that has to be formatted
          model.write(System.out);
          break;
      case Query.QueryTypeDescribe:
          model = qe.execDescribe(); // Result that has to be formatted
          model.write(System.out);
          break;
      case Query.QueryTypeSelect:
          ResultSet results = qe.execSelect(); // Result that has to be
                 // formatted
          ResultSetFormatter.outputAsTSV(System.out, results);
          break;
      }

      model.close();
      qe.close();

    }

}

The mentionend .rdf file can be downloaded here: http://jena.apache.org/tutorials/sparql_data/vc-db-1.rdf

Answer 1

I finally found an answer:

I take a ByteArrayOutputStream and use the various model.write() functions do the rest. For the SELECT-Query I use a textual representation because the provided ResultSetFormatter.outputAsRDF() gives me way to much nonsense back. Anyways, here’s the relevant code

ByteArrayOutputStream baos = new ByteArrayOutputStream();
...
int queryType = q.getQueryType();
switch (queryType) {
case Query.QueryTypeAsk:
    boolean b = qe.execAsk();
    ResultSetFormatter.outputAsRDF(baos, notation, b);
    retval = baos.toString();
    break;
case Query.QueryTypeConstruct:
    model = qe.execConstruct();
    model.write(baos, notation);
    retval = baos.toString();
    break;
case Query.QueryTypeDescribe:
    model = qe.execDescribe();
    model.write(baos, notation);
    retval = baos.toString();
    break;
case Query.QueryTypeSelect:
    ResultSet results = qe.execSelect();
    ResultSetFormatter.out(baos, results);
    retval = baos.toString();
    break;
}